Glass reinforced, epoxy resin impregnated laminates, that retain good flexural strength at temperatures up to 200.degree. C, are needed for slot liners and wedges in large rotating apparatus, and as copperclad printed circuit boards. The anhydride 3,3',4,4',-Benzophenone-tetracarboxylic dianhydride (BTDA), when used as a curing agent for bisphenol A type epoxy resins, has been known to impart superior high temperature resistance to the cured resin systems. Its use so far has been hampered, however, because it is not soluble to any extent in common solvents nor in epoxy resins, except at elevated temperatures where reaction occurs causing premature gellation of the resin system.
Barie and Franke, in I & Ec, Prod. Res. & Dev. Vol. 8, pg. 72, March 1969, "High Temperature Epoxy Resins Based On BTDA", describe uncatalyzed, BTDA hardened-epoxy resin pastes, with anhydride to epoxy equivalent ratios between 0.6 and 0.9 and a curing temperature of 200.degree. C, for use as a metal adhesive, which can also contain filler particles. They also describe uncatalyzed, BTDA-maleic anhydride-epoxy resin casting compositions, with anhydride to epoxy equivalent ratios between 0.5 and 0.95, mixing temperatures of 150.degree. C and a pot life of 5 to 10 minutes. These compositions having a curing temperature of about 200.degree. to 220.degree. C, and flexural strength values (ASTM D-790), after a 24 hour 200.degree.C cure, of 13,200 psi after 5 weeks (A/E ratio 0.85). The BTDA-maleic anhydride is added to hot epoxy resin with stirring for 10 minutes until the BTDA goes into solution. These compositions have been suggested for use in high temperature laminates, but the high press temperatures necessary to cure, and the very poor pot life and/or prepreg shelf life would make the system commercially unfeasible. U.S. Pat. No. 3,794,555, assigned to the assignee of this invention, discloses BTDA-NADIC methylanhydride-epoxy resin solutions which are admixed at temperatures over 100.degree. C to provide moderate BTDA inclusion. There is a need then for improved BTDA-epoxy systems with improved pot life and/or prepreg shelf life and very high BTDA loadings.